Such a scanning device is known from, for example, German patent document 26 09 335 C2. That device has an axial-pressure bearing with plane ball-guiding surfaces between a shell which houses that shaft bearing which is nearer to the head wheel disk, and an element for axially fixing and radially adjusting the shell. Three adjusting elements are distributed around the periphery of the tape-guiding drum, of which at least one adjusting element is provided with a plug that is longitudinally shiftable by means of a threaded pin which is beveled at one end and cooperates with a phase of the shell.
U.S. Pat. No. 4,641,214 discloses a rotary head device in which a shaft is clamped between two fixed and coaxially disposed drums. A rotary body for holding the magnetic heads is mounted on the shaft by means of two ball bearings spaced apart from each other. Both of the above-mentioned devices require ball bearings of the highest precision. In order to limit the play between the inner and outer ring of a ball bearing, it is known to cause the inner ring to bear against the outer ring by means of a limited spring effect provided on the inner ring. In the case of two ball bearings disposed at a certain distance from each other, this effect is obtained by providing that respective inner rings of both ball bearings are longitudinally shiftable on the shaft, while along each of the inner rings a compression spring is provided to provide the desired bearing race bias force. There is a disadvantage, however, that thermally caused changes or wear of the ball races produce height fluctuations of the magnetic head orbit.
It is further known from U.S. Pat. No. 4,603,359 to provide a rotary head device with a fixed lower drum and a rotary upper drum which carries the rotating magnetic heads. In parallel surfaces of the lower and upper drum, ring-shaped grooves or traces are provided for seating an axial ball bearing. This known head device is subject to radial shock and is not suited for scanning oblique tracks which are only a few microns wide.